Torrential rain is forecast to continue in Vietnam's biggest coffee growing provinces in coming days, government reports said on Friday, potentially further disrupting harvesting of the 2016/2017 crop.

That has been stoking worries about supply from the world's top producer of the robusta variety of coffee, with rain already hitting harvests earlier this week, while drought earlier in the season could also crimp output.

Prices for robusta, often used to churn out cheaper instant coffees, firmed on Thursday following news of rain in Vietnam.

Up to 150 mm (six inches) of rain is forecast on Friday and Saturday in the provinces of Dak Lak and Lam Dong, Vietnam's biggest coffee growing provinces, and also in south-central coastal provinces, the national weather center said in a bulletin.

Floods have emerged in several districts in Dak Lak and could rise further, the state-owned center said.

The Central Highlands produces around 80 percent of Vietnam's total coffee output. Coffee trees planted on hills are often safe from flooding, but rain can delay harvesting and affect bean-drying.

Provincial authorities in the central and southern regions should monitor the weather, dams and infrastructure projects to ensure the safety of local people, the government said.

Floods following heavy rain returned to central provinces on October 30, killing at least four people, another government report said on Thursday.

Floods last month killed dozens of people in Vietnam's central region and inundated more than 100,000 houses.